"The intriguing part of this is the kind of tumours we saw were similar to tumours noted for quite some time in some epidemiological studies in heavy duty cellphone users", John Bucher, a senior scientist with NTP, said in a telephone interview.

The results, he says have not led him to change his own cellphone use or to urge his own family to do so. But there was overall little difference in the actual health outcomes of exposed mice and rats compared to their untouched counterparts by the experiment's end - some control groups even died sooner than those exposed to the radiation. "It's important to understand that â€" as is commonly done in these types of risk assessment studies â€" the study was created to test levels of radiofrequency energy exposures considerably above the current safety limits for cell phones to help contribute to what we already understand about the effects of radiofrequency energy on animal tissue.

"International and USA organizations and health experts have maintained their longstanding conclusion that the scientific evidence shows no known health risk due to the RF energy emitted by cellphones", said Justin Cole, a spokesman for the trade group. "These studies should have been done before more than 90 per cent of us, including children, started using this technology day in and day out". The increased risk occurred only in male animals exposed to the highest levels of radiofrequency radiation, which exceeded the exposure levels associated with typical cell phone usage.

The long-awaited results of a $25 million National Institutes of Health study on the effects of cellphone radio frequency radiation exposure on animals is out, and the results are mixed.

The concern with this type of radiation is that it produces energy in the form of heat, and frequent exposure against the skin could alter brain cell activity, as some studies have suggested. But this was not visible in female rats, or in mice.

The Food and Drug Administration, meanwhile, has been aggressively dismissive of any link, saying in no uncertain terms that the "weight of scientific evidence does not show an association between exposure to radio frequency from mobile phones and adverse health outcomes". "Even with frequent daily use by the vast majority of adults, we have not seen an increase in events like brain tumours".

Asked what the public should take from the study, Bucher said, "I wouldn't change my behaviour based on these studies, and I haven't".

"The current safety limits for cell phones are acceptable for protecting the public health", FDA radiation health chief Dr Jeffrey Shuren said in a statement. It released two draft reports on its extensive, expensive efforts to study the effects of mobile phone radiation on both mice and rats. We have reviewed the 2016 interim NTP results and are now reviewing the full set of data from the NTP draft final report.

Samsung and Apple did not immediately respond to requests for comment.